The present invention relates to a device for retaining caps, in particular crown caps, in a machine for forming the sealing gasket inside the caps.
Machines which comprise two carousels which rotate about vertical axes are currently known for forming the gaskets in crown caps.
In one of the carousels, a dose of plastic material in the pasty state is introduced in the caps and is then compressed in the other carousel, by means of appropriate punches so as to form gaskets which line the bottom of the caps and ensure tightness when the caps are applied to the containers.
In order to make the dose of plastic material first and then the gasket adhere to the inner surface of the caps, said caps are heated beforehand so as to liquefy the inner priming coat which acts as adhesive between the gasket and the cap.
Heating occurs by conveying the caps through a magnetic field emitted by a high-frequency inductor. Depending on the nature of the material with which the caps are manufactured, attraction toward the inductor occurs if the material is ferromagnetic or repulsion occurs if the material is diamagnetic, for example aluminum or stainless steel. In order to conveniently increase the efficiency of the inductors, the surface of the caps must pass very close to said inductors without however touching them, in order to avoid short circuits or abrasions of the paint which covers them externally.
For the transfer of the caps from one carousel to the other or for the insertion or removal of the caps in or from the carousels, as well as for the retention of the caps in the carousels, said caps rest toward the inside of appropriate seats defined on the peripheral region of rotating disks (so-called stars) which form an integral part of the carousels and rest outward on fixed guides which extend concentrically with respect to said disks.
Above the caps there is a cover which has the function of providing a better guiding of the caps during advancement, especially if said caps are made of diamagnetic material, which would cause their upward repulsion.
Known cap supporting systems have some severe disadvantages which can be ascribed to the relative movements between the rotating disks and the stationary guides, which cause the wear of the outer guides, produced by the friction of the caps as they advance, and of the seats of the stars, since the advancement of the caps causes the rotation thereof in their seats. These wear phenomena are furthermore aggravated by the magnetic attraction and repulsion force.
Another disadvantage which can be observed in conventional cap conveyance systems is constituted by sparks, which are due to electric discharges which occur in the points of contact between the caps and the fixed guides and are due to currents induced by the magnetic field. Said sparks lead to the wear of the guides and to the damage of the caps.
The accommodation of the caps in the respective seats and their guiding furthermore entail a certain play which, considering the size tolerances of the caps, can cause a misalignment thereof with respect to the punches and therefore centering errors in the formation of the gasket.
A further and not less important disadvantage occurs during the passage of the caps from one carousel to the other, since guides and covers are provided in order to prevent the caps from being left loose during this passage, constituting a constructive complication and being in any case the source of jammings when dimensional deformations or alterations of the caps are added to the negative effects of said plays.
Another disadvantage of conventional conveyance systems is observed in the presence of defective caps which, since they cannot be removed automatically as they are obstructed by the cover, can only be removed manually by stopping the machine.